3.x: Fix diamonds, spelling, unnecessary code

[akarnokd]

• Use diamond where possible
• Remove type arguments where they can be now inferred under Java 8
• Fix spelling errors
• Remove unnecessary throws declarations

[codecov]

Codecov Report

Merging #6804 into 3.x will increase coverage by 0.02%.
The diff coverage is 99.72%.

@@             Coverage Diff              @@
##                3.x    #6804      +/-   ##
============================================
+ Coverage     98.26%   98.28%   +0.02%     
+ Complexity     6378     6376       -2     
============================================
  Files           711      711              
  Lines         46247    46243       -4     
  Branches       6330     6329       -1     
============================================
+ Hits          45443    45449       +6     
+ Misses          287      286       -1     
+ Partials        517      508       -9

Impacted Files	Coverage Δ	Complexity Δ
...3/internal/observers/DeferredScalarDisposable.java	97.91% <ø> (ø)	20 <0> (ø)	⬇️
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	89.9% <ø> (-3.73%)	2 <0> (ø)
.../rxjava3/internal/disposables/EmptyDisposable.java	100% <ø> (ø)	17 <0> (ø)	⬇️
...operators/completable/CompletableToObservable.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...a3/internal/operators/maybe/MaybeFromRunnable.java	100% <ø> (ø)	6 <0> (ø)	⬇️
.../operators/mixed/CompletableAndThenObservable.java	100% <100%> (ø)	2 <1> (ø)	⬇️
...al/operators/flowable/FlowableMapNotification.java	100% <100%> (ø)	2 <1> (ø)	⬇️
...nternal/operators/maybe/MaybeTimeoutPublisher.java	98.55% <100%> (ø)	2 <1> (ø)	⬇️
...3/internal/operators/flowable/FlowableGroupBy.java	95.59% <100%> (ø)	3 <0> (ø)	⬇️
...va3/internal/operators/flowable/FlowableDelay.java	100% <100%> (ø)	3 <0> (ø)	⬇️
... and 264 more

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